Plasma cholesterol and triglyceride are causally related to atherosclerosis, a precursor to heart disease, which is the leading killer in the industrialized world. A complete understanding of cholesterol and triglyceride metabolism and the lipoproteins that carry them in the blood is therefore essential to combat this disease and fulfill the mission of the NIH and NHLBI. A forward genetic screen has identified a family of mice with very low (~25% of normal) cholesterol levels. Analysis of size- fractionated plasma from mutant mice revealed a reduction in the cholesterol and triglyceride content of the two major mouse lipoproteins (HDL and VLDL). The gene responsible for this low cholesterol phenotype has been identified as Mia2. It is a novel player in the metabolism of lipoproteins that apparently works by dramatically reducing their secretion from the liver and/or intestine. This mouse model will be exploited to further the basic understanding of lipoprotein production. This proposal aims to answer some very basic questions as to mechanism of action. Using primary hepatocytes of mutant animals and established tissue culture cell lines, the precise step in the production of lipoproteins that is perturbed by this mutation will be investigated. Secondly, the action of this novel gene in the whole animal will be explored using newly generated tools, including a transgenic mouse line and recombinant adenovirus, which is a very versatile means of expressing foreign genes in a variety of cell lines and tissues and in live mice. These tools will then be used for several experiments, including rescue of the mutant phenotype in vitro and in vivo. Rescue is important in that it proves beyond any doubt that we have identified the mutant gene responsible for the phenotype. Lipoprotein metabolism in rescued mice and cells will then be studied for a full exploration of this novel pathway in lipoprotein production.